The present invention relates to a method for treatment of halogen-containing waste material.
The disposal of halogen-containing waste material, such as waste material containing PVC (polyvinylchloride) and/or other halogen-containing polymers, causes pollution problems, particularly because the combustion of such halogenated organic materials usually results in the release of the halogen atoms in the form of noxious products. For example in the case of chlorinated organics, HCl may be released, which, because of its corrosive nature, is a serious source of environmental pollution.
A large number of methods for treatment of such waste materials are known, such as catalytic cracking methods, hydrogenation cracking methods and pyrolysis methods. The cracking methods as such suffer from the drawback that they can only be used for treating materials having low contents of halogen-containing polymer materials. Further, the cracking method is very expensive, and large acid resistant plants are needed for carrying out the method.
Pyrolysis methods are in general more flexible and can be used for treatment of most waste materials.
EP-A1-0 125 383 discloses a method for decomposition of organic waste material containing halogen by treatment of the waste material in a molten salt pool comprising a mixture of basic alkaline earth metal compound and an alkaline earth metal halide. An oxygen-containing gas is introduced into the pool containing the waste to produce a gaseous combustion product and to cause the halogen present in the waste to react with the basic alkaline earth metal compound to produce additional alkaline earth metal halide.
It is essential that the salt pool is kept in a molten state and the alkaline earth metal is kept in a dissolved state. This means that high temperatures are needed. Further, a large amount of alkaline earth metal is used.
EP-B1-0 111 081 and DE-C1-3 435 622 disclose pyrolysis methods for treatment of halogen-containing waste materials, wherein the waste materials are slowly annealed in a rotating oven at a temperature between 300 and 600.degree. C. Basic compounds, such as CaCO.sub.3 and Ca(OH).sub.2 are added to the waste materials before or after the annealing process in order to neutralize the acids produced in the annealing process. The methods, however, only remove a part of the acids produced, and large amounts of gaseous acids, such as HCl are still emitted to the environment.
WO 91/18960 discloses a method for treatment of PVC waste material, wherein the PVC is subjected to a temperature of between 150 and 300.degree. C., until all halogens are emitted as HCl. The HCl is then collected for reuse. Because of the highly corrosive HCl, this method needs special equipment and is not economically profitable.
Another method for removing the halogen from halogen-containing polymer compounds, which is disclosed in a number of publications of Japanese origin, is based on a treatment of the halogen-containing polymer in an aqueous medium, typically basic solution of alkaline earth metal hydroxide or soil, at an elevated temperature of 160-300.degree. C.
Thus JP-A-74-112979 discloses a process for removing the halogen contained in a polyhalogen polymer, comprising the steps of dispersing a polyhalogen polymer into an aqueous medium containing one or more selected from alkali metal hydroxides, alkaline earth metal hydroxides, salts of alkali or alkaline earth metals, iron and its compounds, zinc and its compounds, aluminum and its compounds, and sulfur, heating the mixture in the presence of an inactive gas or oxygen under pressurization, for removing the halogen or hydrogen halide.
It is stated that the known treatments for removing the halogen or hydrogen halide from polyhalogen polymers performed in a vapour phase in a nitrogen or oxygen atmosphere is difficult to control, if a recovery of halogen-free polymer is the object, and this problem is alleged to be solved by the proposed hot water treatment.
A very similar process is proposed in JP-A-74-16049, where PVC is heated at 160-300.degree. C. in basic aqueous solutions containing alkali and/or alkaline earth metal hydroxides and optionally catalytic amounts of organic amines.
In a very similar earlier proposal by the same applicant, viz. DE 22 60 393 B2 a process for dehydrohalogenation of PVC is disclosed, wherein PVC is heated in an aqueous solution of an alkaline inorganic material selected from ammonia, alkali or alkaline earth metal hydroxides and optionally a catalytic amount of an ammonium salt and/or ammonia at 180-300.degree. C.
The reaction is carried out by adding PVC to a hot alkaline aqueous solution and treating the medium at 180-300.degree. C. This temperature is critical for the quality of the dehalogenated end product. In order to avoid evaporation of the aqueous medium an elevated pressure is used which depends on the dissolved inorganic material.
The disadvantage of the processes based on the treatment in an aqueous medium is related to the evaporation of the water at the high reaction temperature which either has to be countered by addition of an active gas or oxygen or results in elevated pressures which are difficult to control.
Also the high amounts of water which are used according to the above-mentioned citations requires a substantial reactor volume in comparison with the volume of the treated waste material.